Fluorescent compounds can be covalently or noncovalently attached to other materials to impart color and fluorescence. Bright fluorescent dyes permit detection or location of the attached materials with great sensitivity. Certain carbocyanine dyes have demonstrated utility as labeling reagents for a variety of biological applications, e.g., U.S. Pat. Nos. 4,981,977; 5,268,486; 5,569,587; 5,569,766; 5,486,616; 5,627,027; 5,808,044; 5,877,310; 6,002,003; 6,004,536; 6,008,373; 6,043,025; 6,127,134; 6,130,094; and 6,133,445; WO 97/40104, WO 99/51702, WO 01/21624, and EP 1 065 250 A1; and Ozmen et al., Tetrahedron Letters, 41:9185 (2000). Nevertheless, many carbocyanine dyes are known to share certain disadvantages, e.g., severe quenching of the fluorescence of carbocyanine dyes in biopolymer conjugates, and quenching of Cy5 and Cy7 dye variants on conjugates, as discussed in Gruber et al., Bioconjugate Chem., 11:696 (2000), and in EP 1 065 250 A1. In addition, certain desired sulfoalkyl derivatives of the reactive carbocyanine dyes are difficult to prepare, such as Cy3 and Cy5 variants (see Waggoner et al., Bioconjugate Chem., 4:105 (1993)). Cyanine dyes also have a very strong tendency to self-associate (i.e., stack), which can significantly reduce the fluorescence quantum yields (Mishra et al., Chem. Rev., 100:1973 (2000)). Accordingly, new labeling reagents are needed to aid research in the field of biotechnology.